Your tax liability is what you actually owe to the government after all credits, deductions and dependents are factored in (but not counting self-employment tax). For example, if you had $8000 of job withholdings and got a $1000 refund, your tax liability was $7000. Or, if you had $8000 of withholdings and owed $1000, your liability was $9000.
If your income tax liability is expected to be less than $7500, the only way to increase your tax liability is to have fewer deductions or more income. For example, you might temporarily stop your contributions to a pre-tax IRA or 401k and contribute to a Roth IRA or Roth 401k instead. That would remove a deduction and raise your taxable income, while still giving you tax-free benefits when you retire.
You can reduce your withholding if you want to get more money in your paychecks in return for a smaller refund later, but changing your withholding does not change your tax liability.
